I figured out the problem, but you won't believe what it was. In retrospect, it would seem like such an obvious thing to check, but hindsight is always 20/20.
The problem was simple. Instead of following the instructions at
https://github.com/Sigil-Ebook/Sigil..._edge_Linux.md, I followed the ones at
https://github.com/Sigil-Ebook/Sigil...ng_on_Linux.md. That's all. Because I was trying to build on a Mate 18 system, which is considered "cutting edge," I had to use the new compilation instructions, which did not exist the last time I had to compile Sigil.
It seems like a rookie mistake, but I had already seen the documentation at GitHub before and even saved the link, so I didn't consider the possibility that new instructions had been added. I'm certainly not blaming anyone except myself for the mistake, considering the prominent notice at the top of the old instructions pointing to the new ones. I scrolled right past it because I had no reason to think the top of the document had changed.
Everything works flawlessly now.